The use of dendritic cells in cancer immunotherapy is presently an area of significant clinical inquiry. Dendritic cells are highly effective in presenting antigens to responding T-cells; however, dendritic cells normally constitute less than one percent of blood mononuclear leukocytes. Accordingly, a number of in vitro methods have been developed to expand populations of dendritic cells to augment anti-cancer immunity. By exposing increased numbers of dendritic cells to antigens on tumor or other disease-causing cells, followed by reintroduction of the antigen-loaded dendritic cells to the patient, presentation of these antigens to responding T-cells can be enhanced significantly.
For example, culturing blood mononuclear leukocytes for eight days in the presence of granulocyte-monocyte colony stimulating factor (GM-CSF) and interleukin-4 (IL-4) produces large numbers of dendritic cells. These cells can then be externally loaded with tumor-derived peptide antigens for presentation to T-cells. Alternatively, the dendritic cells can be transduced to produce and present these antigens themselves. Expanding populations of dendritic cells transduced to produce and secrete cytokines which recruit and activate other mononuclear leukocytes, including T-cells, may be an even more effective method of generating anti-tumor immune responses.
Transducing cultivated dendritic cells to produce a particular generic tumor antigen and/or additional cytokines is labor intensive and expensive. More importantly, this procedure likely fails to produce and present those multiple tumor antigens that may be most relevant to the individual's own cancer. Several approaches have been proposed to overcome this problem. Hybridization of cultivated autologous dendritic cells with tumor cells would produce tetraploid cells capable of processing and presenting multiple unknown tumor antigens. In a second proposed approach, acid elution of Class I and Class II major histocompatability complexes (MHC) from the surface of malignant cells would liberate a broad spectrum of tumor-derived peptides. These liberated peptides could then be externally loaded onto MHC complexes of autologous cultivated dendritic cells.
Conventional photopheresis is a method of vaccinating patients against leukemic lymphocytes, even when the distinctive tumor antigen(s) is not known. In this method, malignant cells are exposed to photo-activated 8-methoxypsoralen (8-MOP) which enhances cell surface display of Class I MHC-associated tumor antigens. After intravenous return of these altered malignant lymphocytes to the original patient, a potent anti-tumor response may be generated in about 25% of the patients, leading to diminution of the malignant cell population and occasionally long-standing remissions. Experimental studies in mice, in which autologous dendritic cells are first grown in tissue culture and then admixed with the 8-MOP-treated tumor cells, appears to increase the efficacy of conventional photopheresis. In this experimental protocol, tumorigenic mouse T-cells are rendered apoptotic by photopheresis using 8-MOP and exposure to ultraviolet (UV) energy. Following this chemical alteration of the malignant leukeocytes, autologous cultured dendritic cells are added to the apoptotic T-cells, and the cell mix is incubated overnight with shaking to maximize contact between the T-cells and the dendritic cells. The apoptotic T-cell/dendritic cell mix has proven to be an effective cellular vaccine in test mice challenged with viable tumorigenic 2B4.11 cells.
While the above-described experimental protocol is apparently more efficient and comprehensive than alternative approaches, it requires extensive ex vivo cellular manipulations over a period of several days. Accordingly, an in vivo procedure which could in a single day provide large numbers of functional dendritic cells and expose those cells to apoptotic tumor cells would greatly simplify the means by which the anti-tumor cellular vaccine could be prepared.